In general, a transformer of the type with which the present application is concerned comprises a magnetic core having a plurality of leg pieces and yoke pieces connecting the leg pieces to form a generally rectangular flux path surrounding a window. In the case of a three phase transformer, the magnetic core will comprise three leg pieces and four yoke pieces and will have two core windows. Supported on each of the leg pieces will be a coil having a high voltage winding and a low voltage winding each comprising one or more layers of aluminum or copper conductor wound around a coil window that is dimensioned to be mounted on the respective core leg. Electrical connections are made to the high voltage and low voltage windings to accomplish the desired step up or step down in voltage between the input and output.
From the standpoint of cost, it is highly desirable to achieve an output of the transformer, which is typically expressed in kilovolt-amperes (KVA), with a minimum of material. The output in terms of kilovolt-amps from a transformer is defined by the following formula: ##EQU1## Where f=frequency, hz
Basically, the flux density is the amount of flux per cross sectional area flowing through the core, the current density is the amount of amperage per cross-sectional area flowing in the wound conductor in the coil, and the space factors are measures of the utilization of the space within the core and coil windows. More specifically, the coil space factor is a measure of the utilization of the space within the core window by the coil, and this factor is maximized when all of the available space within the core window is either conductor or layer insulation. The core space factor is the measure of the utilization of space within the coil window and would be maximized if all of the space in the window is occupied by the core leg and the core insulation.
Since the frequency is established at 60 hertz, the KVA output per parts size of the transformer is maximized when the flux density, current density and space factors are maximized. Conversely, improvement in these factors will enable the physical size of the transformer to be reduced for a given KVA output rating because of better utilization of the magnetic core and coil material.
As indicated above, one way to increase transformer output or achieve the same output at a lower material requirement is to improve the flux density in the magnetic core. In grain oriented steels used in magnetic core laminations, there is minimum hysteresis loss and reluctance when the direction of magnetic flux through it coincides with the direction of grain orientation, that is, with the lengthwise dimension of the strip, and there is a maximum loss and an increase in reluctance when the flux direction is at right angles to this direction of grain orientation. Oriented magnetic steels all saturate at approximately 130 kilolines per square inch in a direction parallel to the grain orientation, and saturate at approximately 90 kilolines per square inch in directions perpendicular to the grain. In a core having laminations of an alternately laid up E and I configuration, one-half of the yoke lamination stack will saturate at 90 kilolines per square inch and the other half will saturate at 130 kilolines per square inch thereby rendering an overall saturation limit of approximately 110 kilolines per square inch. Allowing for a 10% overvoltage, the maximum operating density is approximately 100 kilolines per square inch. In both the E and I configuration and in a butt-lap lay of core the resistance to flux flow at a core corner, where the flux must flow from a lamination piece in a direction parallel to the grain orientation, across a joint or gap and then into an adjoining lamination which is oriented 90.degree. to the first laminations results in a "corner effect" which reduces the overall saturation limit of the joint and produces vibrations resulting in noise. In order to overcome this problem, prior art magnetic cores in oil filled transformers comprise laminations wherein the corner joints are mitered or beveled in order to minimize the cross grain travel of the flux in the corners of the core. However, the magnetic cores in most dry-type transformers have employed butt joints wherein the lines of flux in adjoining laminations flow in direction 90.degree. to the grain orientation around the corners of the core.
As indicated above, a second factor in improving the output of a transformer without increasing its size, or, alternatively, maintaining the same output with a smaller size, is to improve the coil space factor within the core window and the core space factor within the coil window. Typically, the coils are wound around a rectangular mandrel or form, and the conductor is tensioned between the mandrel and conductor supply so that each layer of conductor can be tightly wound on the preceeding layer and duct spacers. After winding, however, when the tension on the conductor is released, springback in the conductor tends to cause the coil to bow outwardly in the side portions thereof thereby producing convex crowning on the exterior surfaces of the coil sides and concave crowning on the interior surfaces thereof, which are immediately adjacent the core.
In a transformer where each core window is occupied by the side portions of one or two adjacent coils, this crowning effect necessitates that the core window be larger than would otherwise be necessary in order to provide sufficient room for the coil or coils. Overbuilding of the core to provide a larger core window results in less than maximum utilization of the core window thereby decreasing the space factor. Typically, prior art dry-type transformers have a coil space factor within the core window of 55% caused by breathing space, overbuilding and the spring back effect discussed above. The springback also causes a slight bowing out of the inner conductor layers as well, thereby providing a slight concave space between the inner conductor layer of the coil and the core leg, thereby resulting in a less than optimum space factor for the core within the coil window.
The third factor which limits the output of a transformer is the current density within the coil. Heat buildup inside the copper or aluminum conductor of a transformer dictates that a short circuit or severe overload such as fifty times normal current for two seconds and/or two times normal current for thirty minutes will cause the conductor to melt. In order to drive the current density as high as possible, it is necessary to conduct heat away from the conductor to the ambient so that the temperature of the conductor will stay within acceptable limits. As the cooling of the conductor within the coil is increased, the current density can be concomitantly increased thereby resulting in an increase in the KVA output of the transformer.
Typical prior art dry type transformers are rectangular in shape with the conductor layers in the side portions in close overlapping relationship and most or all of the conductor layers in the end portions being spaced apart so as to form air ducts therebetween to permit air to flow through the conductor layers thereby conducting heat away from the coil. Although the temperature of the conductor within the coil end portions can be maintained at an acceptably low level quite easily due to the presence of the air ducts, there has been a problem in conducting heat away from the tightly wound layers in the sides of the coil. A portion of the heat is conducted inwardly to the core, which functions as a large heat sink, but the majority of the heat must be transmitted down to the air ducts in the ends of the coil for dissipation into the ambient air surrounding the coil.
In order to space apart the conductor layers in the ends of the coil, duct spacers of various types have been used in the past. Basically, duct spacers are elongate elements made of a material which is not electrically conductive, such as a glass filled high temperature polyester. In oil filled transformers, there are a series of closely spaced duct spacers within each duct, and because the oil surrounding the coil is such an effective conductor of heat, the problem of providing sufficient breathing space within the ducts is not nearly the problem that it is in air cooled dry type transformers wherein maximum exposure of the conductor layers to air is such a high priority. In prior art dry type transformers, the air ducts in the ends of the coil are formed by inserting elongate duct spacers between adjacent conductor layers during winding of the layers, and by locating the duct spacers at the corners of the conductor layers so that as the next layer is wound thereon, it will be bent along the duct spacers to form corners and will be spaced from the preceeding layer by the duct spacers. Although locating the duct spacers at the corners of the coil is useful to space the end conductor layers the entire width of the coil, and to maintain the structural integrity of the coil after winding to prevent collapsing of the coil during further assembly of the transformer and during use, particularly under short circuit conditions, the corner duct spacers act as thermal barriers inhibiting the flow of heat from the sides of the coil to the air ducts in the ends. The heat generated within the tightly wound sides of the coil tends to flow along the conductor layers toward the cooler end portions of the coil and the corner duct spacers act to insulate the corner portions of the conductor layers from the ambient thereby maintaining the corners at relatively high operating temperatures, which impedes the flow of heat from the coil sides past the conductor layer corners. The inability to more efficiently conduct heat away from the transformer coil imposes a constraint on the maximum current density for the coil, thereby, necessitating more conductor to achieve the same amount of flux.
Prior art dry-type transformers have less than optimum flux density, current density and space factors due to the deficiencies outlined above. For example, the flux density in the core is typically in the 90 to 100 kilolines per square inch range, the current density is approximately 1200 amps per square inch, and the coil space factor within the core window is approximately 55%. This gives a utilization of these three factors in prior art transformers which directly translates into requiring a large core, high conductor requirements to achieve the desired current rating and high noise levels due to the larger physical size of the transformer.